1. Field of the Invention
The invention relates to a new device for amplifying and sampling high dynamic signals.
The device of the invention is adapted in particular to the amplification and sampling of signals in a signal acquisition chain, before digitization thereof in an analog-digital converter and recording thereof, and in particular to the amplification of seismic signals.
Seismic prospection methods generally include the transmission of waves by a seismic source, reception of the waves reflected back by the different reflecting interfaces of the subsoil and the formation of seismic recordings representative of the configuration of the subsoil. The waves are received by reception devices having a very large number of sensors spaced apart along the seismic profile to be studied. For seismic prospection at sea, the sensors, are distributed all along and inside a flexible sealed sheath of great length which is called a streamer.
2. Description of the Prior Art
In recent streamers, the assembly of sensors is divided into a plurality of subassemblies. The signals picked up by the sensors of the same subassembly are connected by an electronic data acquisition apparatus which digitizes them and records them. After each transmission-reception cycle, the signals recorded by the different apparatus of the streamer are transmitted to a central control and recording laboratory through a multiplexed transmission system. This includes several transmission lines to which the different apparatus are connected intermittently and sequentially, under the control of the central laboratory. Such a transmission system is described for example in the French Pat. No. 2.471.088.
So as to obtain an ever higher power of resolution, seismic streamers are developed which are longer and longer, containing an increased number of seismic sensors and consequently the number and complexity of the acquisition apparatuses used for collecting all the signals received are correspondingly increased. Acquisition apparatus are disposed generally in sealed boxes inserted between sections of the streamer, these boxes being connected to the central laboratory by data transmission lines, order and test signal transmission lines and also electric power supply lines.
Increase in the collecting capacity of the acquisition apparatus leads to increasing accordingly the number of their electronic modules. This results in an increase in the volume of the inserted boxes and of their weight, and also in a considerable increase in the electric energy consumed. It is then desirable to use acquisition apparatus whose different electronic modules are optimized for reconciling at one and the same time performance, reliability and a relatively low electric consumption. This is true particularly for the amplification chains adapted for bringing the amplitude of the signals received by the seismic sensors to a sufficient level for application to an analog to digital converter.
Amplifier arrangements are known in which the signals received are applied to the inputs of an analog multiplexer disposed at the head of the acquisition chain after previous application in a fixed gain preamplifier. The output of the multiplexer is connected to a single amplifier formed by a chain of amplifier elements disposed in series, the output of each of them being connected to the input of the next one. The gain of each amplifier element is fixed and chosen preferably equal to a whole power of the number 2.
Since the variation in amplitude between any two successive samples may be very large, a selection member chooses the number of amplification units which each signal must pass through so as to reach an optimum level and switches the output of the amplifier element where this signal is available to the input of an analog to digital converter, which is connected to a recording system. In a first variant, described for example in the French Pat. No. 2.110.758, all the amplification elements have an identical gain. In a second variant, the gains of the different amplifier elements of the chain are different from each other and their values are chosen from the successive powers of the number 2. One embodiment is described in the French patent application No. 2.373.914 which relates to an amplifier formed by n stages, the value of the gain of any one amplification stage being equal to the square of the value of the gain of the preceding amplification stage.
Successive samples delivered by the head multiplexer are stored in a sample-and-hold unit (hereafter S & H unit) disposed upstream of the amplification stage and which thus operates on high dynamic signals. It is known, that such an arrangement has a drawback: the S & H units currently used introduce diaphony between successive samples. Errors which result therefrom are amplified with a gain equal to the maximum gain of the amplification chain. Furthermore, buffer amplifiers generally included in S & H units further increase the number of amplification stages.
The device of the invention allows high dynamic signals coming from a multiplexer to be amplified and sampled, while avoiding the above mentioned drawbacks.